1. Field of the Invention
The present invention relates to the field of photography and, more particularly, to photographic film assemblages and photographic apparatus with which the assemblages are adapted to be used.
2. Description of the Prior Art
An important step in the processing of a self developing film unit is the spreading of a liquid processing composition between and in contact with an exposed photosensitive element and a superposed image receiving element to initiate a diffusion transfer process. In order to achieve an optimum quality positive print, it is desirable that the processing fluid be spread such that it covers the entire photoexposed area in a thin uniform layer.
A typical film unit includes a photosensitive element, an image-receiving element which may be superposed on the photosensitive element subsequent to exposure or may be transparent and predisposed on the photosensitive element such that exposure may be made through the image-receiving element, and a rupturable pod or container of processing fluid located at one end of the two elements.
In a typical self-developing photographic system, the film units are arranged in stacked relation within a film container which is adapted to be inserted into the receiving chamber of an appropriate camera to locate the forwardmost film unit in the stack in position for exposure.
Subsequent to exposure, the forwardmost film unit is extracted from the film container and is advanced, pod first, between a pair of pressure applying members mounted within the camera. The pressure applying members exert a compressive force on the pod causing it to rupture and discharge the fluid between the exposed photosensitive element and the superposed image-receiving element. Continued advancement of the film unit between the pressure applying members results in the fluid being advanced along a liquid wave front toward the trailing end of the film unit such that it is progressively distributed over the photoexposed area of the film unit.
The uniformity of the liquid layer is, to a large degree, determined by the initial shape of the liquid wave front. In order to uniformly spread the processing composition over a substantially rectangular or square photoexposed area it is preferable that the wave front be disposed in a substantially straight line which extends outwardly to the lateral margins of the area and is oriented in a direction that is normal to the direction of film advancement between the pressure applying members.
There are several factors which effect the initial shape of the wave front. One is a design of the pod and its rupture characteristics. Another relates to the viscosity and amount of liquid enclosed by the pod. The wave front shape is also influenced by the velocity at which the film unit is advanced through the pressure applying members, the amount of compressive pressure exerted on the film unit, and the resistance to fluid flow at the interfaces between the liquid and the superposed film unit elements.
One of the most commonly observed spread shapes is a tongue shape wherein the wave front progresses more rapidly at the central portion of the photoexposed or image-forming area than out at the lateral margins. This condition may be caused by an uneven distribution of liquid upon initial discharge from the pod, i.e., more liquid being concentrated at the center of the film unit than out at its edges. In the subsequent spreading of the tongue-shaped wave front, it is possible that the corners of the image-forming area at the trailing end of the film unit will be coated with a layer of fluid of reduced depth or thickness, or not be coated at all.
One method employed to compensate for a tongue-shaped wave front has been to provide excess fluid in the pod. Another method has been to equip the camera with spread control devices which serve to modify the shape of the liquid wave front during spreading.
The spread control devices are designed to apply a second compressive force to the central portion of the film unit in the path of the mass of fluid discharged from the pod by the pressure applying members. This serves to retard the central portion of the wave front and cause a flow of liquid in a direction transverse to the direction of film advancement. In this manner, the wave front is modified such that it is substantially straight and is oriented in a direction substantially normal to the parallel sides of the rectangular or square image-forming area.
For example of cameras which are equipped with devices for controlling the distribution of liquid processing composition, reference may be had to U.S. Pat. No. 2,991,703, issued to V. K. Eloranta on July 11, 1961, and U.S. Pat. No. 3,416,427, issued to J. B. Murphy on Dec. 17, 1968, both of which are assigned to the same assignee as the present invention.